The invention relates to steam generators of the type comprising a pressure vessel containing a bundle of exchange tubes and whose upper portion forms a steam chamber, a ring header having an approximately toric shape located in the vessel, connected to a feed water intake nozzle passing through the wall of the enclosure and having distribution means for discharging feed water into the enclosure, formed by crook-shaped tubes connected to the header along an upper generatrix thereof.
The invention is particularly suitable for use in nuclear power stations which comprise a steam generator having a bundle of U-shaped exchange tubes through which the primary fluid of the reactor passes.
During normal operation of the reactor, the free surface of the feed water in the enclosure is above the nozzle and the header and the water speed in the nozzle is relatively high. Operation is then satisfactory. It may on the other hand be disturbed during different temporary exceptional conditions.
Steam generators have already been proposed for attenuating the disturbances. French patent No. 2333200 (Westinghouse Electric Corporation) describes a generator having upturned J-shaped discharge tubes which permit the header to remain filled with feed water; one at least of the tubes located close to the water intake nozzle, is shorter than the others and prevents steam from being trapped in the header. The exchange tubes of the bundle being in the form of vertical hair pins with an inlet "hot" leg and an outlet "cold" leg, the tubes are spaced apart with a smaller interval in the portion of the header over the hot leg than in the portion of the header over the cold leg. The dissymmetry of distribution of the openings makes this arrangement unfavorable for the fatigue stresses in the tore-shaped header. Manufacture is moreover complicated by the very close arrangement of the tubes above the hot leg.
Another prior art steam generator differs from the preceding one in that the tubes have an intended wall outside the tore-shaped header, which avoids water hammer in the feed water intake pipe. This document does not deal with the problem raised by the presence of hot legs and cold legs cf the exchange tubes.